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Flexible Forming

Thilo Vogel

Flexible forming processes allow for the adaptation and modification of the product geometry with little effort. They enable the economical production of prototypes and small series, an extended geometry portfolio and the production of thickness-optimized semi-finished products.
Therefore they are serving lightweight design and the trend towards shorter product development cycles as well as increasingly individualized products.

The research topics embedded in this cross-sectional area are assigned to different research groups and are presented below.

Flexible Forming of Sheet Metal Components

Many application parts contain elements which are made by flanging or hole flanging operations. The production of these flanging elements by means of Incremental Sheet Forming, short ISF, can be easily integrated into the production process on the Flexible Sheet Metal Processing Center. Thus, the entire process chain of the component production can be carried out in the same clamping. The advantages of this process integration are demonstrated by a component based on the inspection cover of the Airbus A320. First of all, the curved preform of the target geometry is created within a very short period of time by stretch forming. In ISF, a hemispherical forming tool successively forms the remaining areas of the component. After trimming, the high process limits of ISF are finally used to set up the flange and hole flange elements.

In architecture, elaborate facades or roofs with an extraordinary design usually require massive substructures. A resource-efficient alternative for the realization of complex freeform structures was developed in a joint project of the Chair of Structures and Structural Design and the Institute of Metal Forming: By means of targeted tessellation and folding, thin sheets are enabled to create a powerful, self-supporting facade system without needing any substructures. The reproduction of individualized shapes requires a large number of single parts and flexible manufacturing processes. Incremental Sheet Forming allows for the economical production of these facade concepts. The presented facade consists of 140 individual pyramids and 234 individual triangles made of stainless steel. With a weight reduction of 48 % compared to conventional construction methods, the high lightweight design potential opens up new possibilities for modern facade structures.

The 1964 Shelby Daytona Cobra Coupé was not to be seen in the historical racing sport for decades despite its many successes. Back then, only six original Shelby cars existed. The company American Muscle Motorsports & Services wanted to make the dream of bringing this legend back to the racetrack come true. Using the process combination of stretch forming and Incremental Sheet Forming, the Institute of Metal Forming, short IBF, was able to offer a flexible and cost-effective manufacturing technology for the production of the Shelby's car body. Due to the digital process chain developed at the IBF, planning and process design was accelerated considerably compared to a manual production. The subsequent production took place in the Flexible Sheet Metal Processing Center of the IBF and the body was delivered to the customer after a few weeks. So, the Shelby will soon be back on the road in historical racing.

Manufacturing of an outer skin component of a Shelby Daytona Cobra Coupé

Integrated CAx Process Chain

In prototyping and small batch production, conventional manufacturing processes, such as deep drawing, are usually not economically applicable. Flexible processes with low tooling effort, such as stretch forming and Incremental Sheet Forming, short ISF, are a promising alternative to realize parts within a very short time. In addition, the goal "first time right" is pursued to save resources. Therefore, reliable and precise planning tools and models are needed. The integrated CAx process chain, developed at the IBF, enables the process planning in a CAD-CAM environment with corresponding interfaces to FE models and digital image correlation tools. Numerical simulations of stretch forming or ISF provide digital geometries that can be compared with the target geometry. In so doing, iteration cycles during the prototype production are performed virtually while material-intensive and time-consuming experiments are avoided.

Open-die forging is an incremental bulk metal forming process, which is mainly used for the production of long and straight workpieces with a simple geometry as round or square. The production of complex geometries by open-die forging usually requires additional manufacturing steps or can only be realized by a high amount of machining. A new manufacturing approach is based on the idea to realize the production of complex workpieces through superimposed manipulator displacements during a forging stroke. Due to the plastic state of stress, already small superimposed stresses are sufficient to control the material flow towards the intended final geometry. This new forging method was successfully realized for the production of curved and twisted workpieces from steel and aluminium. The general approach significantly increases the range of producible geometries in open-die forging.

Minimizing material and milling costs by near-net shape ring rolling today plays an important economical and environmental role. Industrially feasible ring geometries are exclusively rotationally symmetric, even though applications with varying volume distribution around the circumference exist, e.g. eccentric rings with a nearly linear wall thickness distribution. Depending on size, these parts are produced by milling, closed-die forging or casting processes, while disadvantages in terms of material waste, process flexibility or mechanical properties have to be accepted.
This project aims to further develop the ring rolling process to enable production of near-net shape eccentric ring geometries. Especially for large parts this allows for large material savings without restricting process flexibility or product properties.

Flexible Rolling enables the production of metal strips with a variable thickness in longitudinal direction. Employing such semi-finished products enables the development of load-aligned structural components. Compared to conventional components light-weight design becomes possible and resources can be saved. During the process, the roll gap is adjusted according to pre-calculated trajectories resulting in the desired thickness distribution. Based on research on the layout of a suitable process control Flexible Rolling has made the step to an industrial-scale application at the company Mubea. A wide variety of thickness profiles and materials is produced for various applications such as automotive parts.

Strip Profile Rolling is a modified rolling process allowing for a thickness variation transversal to the rolling direction. Such semi-finished products offer a potential for light-weight design and enhanced use of material. In order to achieve a thickness distribution in transversal direction it is necessary to use narrow rolls. Narrow rolls guide the material flow into the transverse direction and thereby prevent buckling even though there is an inhomogeneous thickness reduction in transversal direction. A multi-pass process can achieve a distinct width of the area with reduced thickness. Besides the investigation of process boundaries, a combination of Strip Profile Rolling and Flexible Rolling has been investigated. A combination of these processes can lead to semi-finished products in both longitudinal and transversal direction.

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